Can LiFePO4 Batteries be Connected in Parallel?

Aolithium prides itself on making rugged and reliable lithium iron phosphate batteries. It's no surprise that we have a lot of battery-related questions.

One of the most common questions is ‘I need more power, can I parallel the cells? What is the maximum number of LiFePO4 cells I can connect in parallel?’

Paralleling cells in a LiFePO4 battery is a way to increase the capacity of the battery (i.e., how long the battery can run on a single charge).

Of course, paralleling a LiFePO4 battery does not mean that your battery can power any device above its standard output voltage, but rather increases the amount of time it can power the device. It is important to note that when charging LiFePO4 batteries in parallel, the increased ampere-hour capacity may require a longer charge time.

Knowledge about parallel connection of LiFePO4 battery

First of all, we should know that when two or more lithium iron phosphate batteries are connected in parallel, the current flowing through each battery cannot be exactly equal.

For example, suppose you are using two 12V 100Ah batteries in parallel. When the battery system is connected to a 50A load, the load on each cell cannot be exactly 25A. One cell can carry a 21A load, while the other can carry a 29A load. This current difference becomes more pronounced when the load is larger.

In addition, as the number of cells in parallel increases, it becomes increasingly difficult to draw power from each cell and push it evenly to each cell. To complicate matters further, each cell has a unique internal resistance and therefore receives/generates power at slightly different rates.

Causes of current imbalance after parallel connection of LiFePO4 battery

Current imbalance between cells is caused by field installation variables. For example, differences in cell and battery manufacturing processes, differences in cell connection resistance, and temperature differences between cells. Click here to learn about the differences in manufacturing processes between Aolithium and SOK and Battleborn batteries.

Even batteries from the same batch may not have the same internal resistance and power capacity. And the difference in current in each parallel branch causes the battery's state of charge (SOC) to diverge. The battery with the best power capacity will be drained faster than the others. The SOC of all batteries will then converge again as the fastest-draining batteries reach a point where they can no longer provide as much power to the system as possible. At this point, the most powerful batteries will be those that initially provided less power but now have a higher SOC.

The conclusion is that the SOC of each LiFePO4 cell connected in parallel will vary between cells for most of the life of the battery system. The difference in SOC between cells means that eddy currents will flow between cells even when the load is disconnected, because they must equalize the SOC state of the parallel cell system. Sometimes the eddy currents between cells can be abnormally high, causing the cells to go into an unpredictable protection mode.

Note that LiFePO4 cells have a very flat discharge curve and once the load is removed, it can take several hours for the cell to equalize the SOC. No matter how long you wait, the cell may not equalize due to diffusion voltage or what is called "surface charge".

Of course, a battery management system (BMS) can handle parallel bypass current imbalance and eddy current runaway, but it does not solve the problem 100%, only bring a relative balance between parallel cells. Once the current in the parallel bypass is higher than expected, it may shorten the battery life.

Considerations for parallel connection of LiFePO4 batteries

Before connecting batteries in parallel, you may need to pay attention to the following matters.

1. Do not mix different brands and capacities of batteries

Make sure the parallel-connected batteries match strictly internal resistance: capacity, voltage, and brand are exactly the same, and you cannot mix old and new batteries. If batteries of different capacities are used together, current imbalance will occur between the two batteries, thus reducing the running time of the battery system. If batteries of different capacities are mixed for a long time, it will eventually lead to short-circuiting of the battery with smaller capacity. A shorted battery can lead to overheating, further affecting the entire electrical system or even damaging the battery.

2. Fully charge the batteries first

Before connecting the batteries in parallel, make sure that all LiFePO4 batteries to be used in parallel are fully charged individually via the LiFePO4 charger. This way the batteries are more or less balanced, thus maximizing your overall usage time.

3. Use the right connection cables

Make sure that your cable sizes (diameters) are suitable for carrying the required current - they should be the same size as your maximum requirements and usually match your inverter.

Therefore, the battery cables should be the same size or larger than the inverter cables. They also need to be the right length to get the best results.

Ideally, make sure that your jumper cables are as short as possible and that the cells are close together and that the cables are of equal length between each cell.

4. Check the main system cable

The main system cable supplying power to the load should be connected to the entire battery pack to ensure that the batteries are charged and discharged evenly for optimum performance.

Also make sure that the main system cable and the cable connecting the batteries are large enough to carry the total current of the system.

5. Connect the batteries

Connect the positive terminals together and perform the same operation for the negative terminals. Then connect the positive terminal of the first cell and the negative terminal of the last cell to the system.

Be careful not to reverse the positive and negative terminals.

Old and new batteries cannot be connected in parallel

When connecting two or more batteries in parallel that are new and have the same capacity and voltage brand, the power pushed between the batteries is balanced.

As the batteries age, their internal resistance gradually becomes unbalanced.

Batteries are known to consume themselves. Even if the battery has been idle (or has only been used once or twice), the circuitry inside the battery, etc., will consume a small amount of power. The cells inside the battery will slowly age over time.

For example, a 100Ah lithium iron phosphate battery purchased a year ago (or six months ago) and a 100Ah battery of the same brand just purchased will have a slightly different internal cell state and the internal resistance of the battery will be different. The two batteries cannot be used in parallel.

Because the new battery has a lower internal resistance than the old battery. The battery with lower internal resistance is somehow self-correcting and will charge and discharge at a higher current than the other battery, aging faster and catching up with similar batteries. In addition, the new battery has a lower internal resistance than the old battery, and the current flows mainly through the new battery, which affects the service life of the new battery.

Can Aolithium LiFePO4 batteries be connected in parallel?

Aolithium batteries can be connected in parallel, but only if the following conditions are met.

1. The batteries have the same capacity.
2. The batteries are produced from the same batch.
3. The batteries are brand new.
4. The batteries are fully charged before paralleling.

This is the only way to ensure that the internal characteristics of the batteries are closer and that they are better connected in parallel. Obviously, if you plan to connect the batteries in parallel, buying 2 batteries at a time is the best option.

Warm tip: Aolithium 12V-4S LFP battery supports 4 series and 4 parallel.

The 4S is great if you're running an OFFGRID 48V system. That way it can run a higher voltage and more amps.

With an RV space is limited and 90% of owners will run 2 batteries max. With the regular 12v 100ah able to do 200Amps for 3 minutes you can run 1 battery and run a 2000watt inverter. The 3 minute limit is perfect to run a small coffee machine, microwave. water heater, etc. That's the main benefit of the AoLithium vs the competition. As with the competition and with the 12v100ah4S you need 2 batteries minimum to do what the regular 12v 100ah can do. The regular 12v100ah you can make a pot of coffee in the morning without turning on the generator with only 1 battery.This is why the regular 100Ah is better for RV'ers.